1. Field of the Invention
The present invention generally relates to the technical field of wet process phosphoric acid. More particularly, the invention relates to the use of filtration and/or clarifying aids to enhance the filtration rate and/or clarity of phosphoric acid slurries produced by the wet process (WPA).
2. Description of the Related Art
The manufacture of phosphoric acid is well known and is the subject of numerous textbooks. An overall view of the manufacture of phosphates and phosphoric acid is treated by Becker in Phosphates and Phosphoric Acids, Marcel Dekker, Inc. 1989; and by Stack in Phosphoric Acid, Part 1 and Part 2, Marcel Dekker, Inc. 1968.
About 90% of the world's phosphoric acid is produced according to the wet process, which is conventionally prepared by acidulating phosphate rock (which contains calcium phosphate) with sulfuric acid to yield a crude, wet-process phosphoric acid (WPA) containing calcium sulfate and/or fluorosilicates as insoluble waste precipitates (gypsum). These insoluble particles are removed by filtration and/or clarification processes prior to concentration and recovery of the acid. Additional solids are generated during concentration of the acid and can be removed via the clarifiers. Each concentration step typically produces more precipitate contaminants.
The wet process phosphoric acid production process is a commercially important process, with the phosphoric acid produced thereby being an important raw material for the manufacture of phosphates for the fertilizer industry. Efficient filtration of crude phosphoric acid from suspended solids after digestion of the ores, as well as efficient clarification of phosphoric acid at various stages, is critical to maintain production. If a plant is limited by filtration capacity, improvement of filtration efficiency can have a huge commercial impact by increasing production without the need for capital investment. For this reason there has been much effort in the prior art to improve the filtration rate of the phosphoric acid with the use of filtration and/or clarifying aids.
For example, as noted in U.S. Pat. No. 4,291,005 to Poulos et al. (1981) and U.S. Pat. No. 4,800,071 to Kaesler et al. (1989), conventional organic flocculants have been applied to agglomerate fine particulate solids to clarify phosphoric acid and to improve filtration rate. The '005 patent discloses methods of settling suspended solids in phosphoric acid product solutions using acrylamide/acrylate polymers. The polymers disclosed consist essentially of a predominant proportion of acrylic acid or acrylate units (˜95 mol %) and a minor portion of acrylamide units (˜5 mol %). The '071 patent discloses sulfonated acrylamide and sulfonated acrylamide/acrylate polymers for aiding filtration in wet process phosphoric acid production.
U.S. Pat. No. 3,644,091 to Naschke et al. (1972) discloses methods for clarifying phosphoric acid using water soluble sulfonated polystyrenes having molecular weights from about 1 to about 40 million.
Several patents to Rey et al. (U.S. Pat. No. 5,173,280 (1992), U.S. Pat. No. 5,211,928 (1993), and U.S. Pat. No. 5,318,707 (1994)) disclose processes for clarifying phosphoric acid or for improving the filtration of phosphoric acid using high molecular weight acrylamide/acrylamido methylpropyl sulfonic acid-type polymers or carboxylic-type polymer having a portion of sulfonic functionality.
However, while the various reagents discussed above may have applicability, and even some merits, in wet process phosphoric acid production, the filtration part of the process still frequently becomes a bottleneck nowadays when either the filter cloth develops fluorosilicate-type scale (which requires cleaning), and/or when the gypsum particles' size and morphology do not allow for efficient filtration.
Frequently, phosphoric acid plants have to shut down to physically clean the scale or replace the filter cloth after less than a week's worth of operating time. The economic impact for the filtration-related issues is substantial, and the industry is in need of a more efficient filtration and/or clarifying aid technology than the existing ones. Since many factors (e.g., solid species, particulate size and distribution, particulate morphologies and ionic strength, concentration, and viscosity of phosphoric acid medium) can affect the performance of such filtration and/or clarifying aids, it is a great challenge to develop high-efficiency aids useful for clarifying phosphoric acid and/or enhancing the filtration rate.
Accordingly, processes that employ high-efficiency filtration and/or clarifying aids to effectively enhance the filtration rate and/or clarity of phosphoric acid would be a useful advance in the art and could find rapid acceptance in the industry.